U.S. patent number 5,637,968 [Application Number 08/141,783] was granted by the patent office on 1997-06-10 for power tool with automatic downshift feature.
This patent grant is currently assigned to The Stanley Works. Invention is credited to Stephen M. Kainec, William L. Naumann.
United States Patent |
5,637,968 |
Kainec , et al. |
June 10, 1997 |
Power tool with automatic downshift feature
Abstract
A method and apparatus for correcting torque overshoot in power
tools wherein torque per angle rate is sensed on each rundown and
the speed of the tool is automatically adjusted to maintain torque
accuracy on higher torque rate joints while maintaining speed on
lower torque rate joints, automatically minimizing tool heating and
maximizing job rate. In the setting of threaded fasteners, joint
rates are defined as high (hard), medium or low (soft). Two torque
points are defined, a first torque point and a second torque point,
which are percentages of the target torque, for example 25% and 50%
respectively. Two angle values are defined corresponding to the
number of degrees of tool spindle rotation measured along the
portion of the torque/angle curve between the first torque point
and the second torque point on typical linear hard and medium
joints. Two final tightening speeds are defined as percentages of
full speed corresponding to the tool system requirements for
accurate shutoff on the two types of joints, hard and medium. A
controller counts the number of angle degrees between the first
torque point and the second torque point. The angle counted is
compared with the values of hard and medium angles to determine the
joint rate. If the angle is less than or equal to the hard angle,
the joint is considered hard and the controller immediately
downshifts the tool to the hard speed. If the angle is greater than
the hard angle but less than or equal to the medium angle, the
joint is considered medium and the controller immediately
downshifts the tool to the medium speed. If the angle is greater
than the medium angle, the joint is considered soft and the speed
is not changed.
Inventors: |
Kainec; Stephen M. (South
Euclid, OH), Naumann; William L. (Chesterland, OH) |
Assignee: |
The Stanley Works (CT)
|
Family
ID: |
22497232 |
Appl.
No.: |
08/141,783 |
Filed: |
October 25, 1993 |
Current U.S.
Class: |
318/432; 173/5;
29/407.01; 29/707; 81/469 |
Current CPC
Class: |
B23P
19/066 (20130101); B25B 23/14 (20130101); B25F
5/001 (20130101); G05D 17/02 (20130101); Y10T
29/5303 (20150115); Y10T 29/49764 (20150115) |
Current International
Class: |
B25B
23/14 (20060101); B23P 19/06 (20060101); G05D
17/00 (20060101); G05D 17/02 (20060101); B25B
023/147 () |
Field of
Search: |
;318/432,433,434
;388/937 ;173/4-7,217,93.5 ;81/467,469 ;73/862.23,862.24
;29/407,707 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sircus; Brian
Attorney, Agent or Firm: Haves & Reinsmith
Claims
What is claimed is:
1. A method for correcting for torque overshoot in an electric
power tool to perform a fastening job wherein the tool is to be
shut off at a target torque, said method comprising:
a) defining first and second torque points which are percentages of
the target torque;
b) defining first and second angle values corresponding to tool
spindle rotation to be measured between the first and second points
for high torque rate and medium torque rate fastening jobs,
respectively;
c) measuring the angle value between the first and second points
during rundown of the tool while performing the fastening job;
d) comparing the measured angle value with pre-defined angle values
for high torque rate and medium torque rate fastening jobs; and
e) utilizing the results of the comparison to perform the fastening
job.
2. The method of claim 1, wherein said step of utilizing the
results of the comparison comprises controlling the speed of the
tool as the target torque is reached.
3. The method of claim 1, wherein said step of utilizing the
results of the comparison comprises:
a) downshifting the tool to a speed providing accurate shutoff in
high torque rate jobs if the measured angle value is less than or
equal to the pre-defined angle value for high torque rate fastening
jobs;
b) downshifting the tool to a speed providing accurate shutoff in
medium torque rate jobs if the measured angle value is less than or
equal to the pre-defined angle value for medium torque rate
fastening jobs; and
c) maintaining the tool speed when the measured angle value is
greater than the pre-defined angle value for medium torque rate
fastening jobs.
4. The method of claim 1, wherein said first and second torque
points are 25 percent and 50 percent, respectively, of the target
torque.
5. A method for correcting for torque overshoot in an electric
power tool to perform a fastening job wherein the tool is to be
shut off at a target torque, said method comprising:
a) defining first and second torque points which are percentages of
the target torque;
b) defining first and second angle values corresponding to the tool
spindle rotation measured between the first and second points for
high torque rate and medium torque rate fastening jobs,
respectively;
c) defining first and second final tightening tool speeds as
percentages of full speed corresponding to requirements for
accurate tool shut off on high torque rate and medium torque rate
fastening jobs, respectively;
d) measuring the angle value between the first and second torque
points during rundown of the tool while performing the fastening
job;
e) comparing the measured angle value with the first and second
defined angle values;
f) downshifting the tool to the first defined final tightening
speed if the measured angle value is less then or equal to the
first defined angle value;
g) downshifting the tool to the second defined final tightening
speed if the measured angle value is less than or equal to the
second defined angle value; and
h) maintaining the tool speed when the measured angle value is
greater than the second defined angle value.
6. The method of claim 5, wherein said first and second torque
points are 25 percent and 50 percent, respectively, of the target
torque.
7. A system for correcting for torque overshoot in an electric
power tool driven by an electric motor wherein the tool is to be
shut off at a target torque, said motor having speed control means
operatively associated therewith, said system comprising:
a) resolver means for providing angle information relating to
degrees of rotation of said tool;
b) torque transducer means for providing torque information
relating to the job being performed by said tool; and
c) controller means connected to said resolver means and to said
torque transducer means for utilizing said angle information and
said torque information to control the speed of said tool, said
controller means comprising means for measuring the angle value
between first and second torque points along a torque/angle curve
of the job being performed by said tool during rundown of said tool
while the job is being performed, means for comparing the measured
angle value with predefined angles for high torque rate and medium
torque rate jobs and means for utilizing the results of the
comparison in connection with controlling said tool during the
job.
8. The system of claim 7 wherein said means for utilizing the
results of the comparison comprises:
a) means for downshifting said tool to a speed providing accurate
shutoff in high torque rate jobs if the measured angle value is
less than or equal to a pre-defined angle value for high torque
rate jobs;
b) means for changing said tool speed to a speed providing accurate
shutoff in medium torque rate jobs if the measured angle value is
less than or equal to a pre-defined angle value for medium torque
rate jobs; and
c) means for maintaining the tool speed when the measured angle
value is greater than the pre-defined angle value for medium torque
rate jobs.
9. A system for correcting for torque overshoot in an electric
power tool driven by an electric motor wherein the tool is to be
shut off at a target torque, said system comprising:
a) means for providing angle information relating to degrees of
rotation of said tool;
b) means for providing torque information relating to the job being
performed by said tool; and
c) control means connected to said means for providing angle
information and to said means for providing torque information for
utilizing said angle information and said torque information to
control the speed of said tool during fastener rundown and prior to
tool shutoff;
said control means including automatic downshift means
comprising:
1) means for obtaining the angle information between two
predetermined torque points on the torque/angle curve for the job
being performed by said tool;
2) means for comparing the obtained angle information to
predetermined angle values for high torque rate and medium torque
rate jobs; and
3) means for downshifting said tool to one of two final rundown
speeds depending upon the results of the angle comparison.
Description
BACKGROUND OF THE INVENTION
This invention relates to power tools and, more particularly, to a
new and improved control system and method to correct for torque
overshoot in such tools.
One area of use of the present invention is in tightening threaded
fasteners, although the principles of the present invention can be
variously applied. In controlled power tool systems used in
tightening threaded fasteners the amount of torque required to turn
the fastener one degree is known as the torque rate. In a typical
electrically operated system, the fastening tool contains a motor
connected through a set of speed reducing, torque amplifying gears,
to any of a number of output heads connecting the tool to the
socket being used to tighten the threaded fastener. At some point
in the gear train a torque transducer is located which generates
electrical signals proportional to the torque being transmitted
through that point in the gearing. The on/off run status of the
tool motor is controlled by a microprocessor-based meter/control
unit.
In such a power tool, an electrical signal proportional to torque
is fed from the torque transducer in the tool to the control unit
in which a torque target has been set. When that target torque is
reached, the run status command signal being sent from the control
unit to the tool is turned off. The tool stops, but not quickly
enough to prevent some torque overshoot above the target torque set
on the controller. The time required to sense the torque, process
the information and remove the run signal, along with the inertia
in the decelerating high-speed elements in the tool, cause the
rotating components to continue to rotate beyond the point at which
they were to have stopped. This excess rotation, when transmitted
through the tool to the fastener, can potentially drive the final
fastener torque well beyond the target torque set in the control
unit.
On a high torque rate threaded joint, the torque can be driven
beyond the upper limit of acceptability for the joint, causing the
joint to be considered unacceptable. The capability of the tool to
control torque on a combined series of threaded fasteners of widely
differing torque rates is a measure of tool accuracy, with minimum
acceptable specifications for particular users. A competitive
advantage exists for manufacturers of tools with the ability to
control torque on such a combined series of joint types. A further
competitive advantage exists for manufacturers whose fastening
systems accomplish this control with a minimum of user involvement
and complexity.
One way to correct the torque overshoot problem is to partially
slow the tool at some point in the tightening sequence earlier than
that point at which the measured torque is equal to the target
torque. This is generally done by employing a downshift feature
where at a pre-defined pre-torque torque value, the speed is
lowered to a specified downshift speed value. The lower speed
proportionally reduces only the portion of the torque overshoot
produced by the excess rotation of the tool motor due to the
inertia in the decelerating high-speed elements in the tool.
One shortcoming of the foregoing approach arises from the fact that
it is convenient for a customer to use fastening systems operated
with a common control unit. The downshift feature described
hereinabove would therefore be active for every tightening
sequence, not just the ones that would benefit from it. The lower
speed is maintained even on joints that require many revolutions to
reach the target torque, which causes delays in production and
excessive electrical tool heating, with no significant improvement
in final torque accuracy. Furthermore, it is inconvenient and
sometimes impossible for a customer to setup these control units
differently for each tool and fastener combination.
Another consideration arising from the foregoing downshift approach
is that the magnitude of the total gear ratio in the tool has a
significant effect on the torque overshoot, given a common motor
and control unit combination. Lower total gear ratios transmit more
of the excess motor rotation to the fastener, while higher ratios
transmit less, making lower ratio tools particularly vulnerable to
torque overshoot. The setting of the downshift parameter values
must therefore be a compromise if low and high ratio tools are to
be operated with a common motor and control unit combination. If
the speed is set to favor the low ratio tools, high ratio tools
using this setup will run slower than necessary causing
aforementioned delays in production and excessive electric tool
heating, particularly on low torque rate joints.
SUMMARY OF THE INVENTION
It is, therefore, a primary object of the present invention to
provide a new and improved system and method to correct for torque
overshoot in power tools.
It is a further object of this invention to provide such a method
and apparatus which operates efficiently and effectively on high,
medium and low torque rate fastening jobs.
It is a more particular object of this invention to provide such a
method and apparatus which allows the tool to run at maximum speed
as long as possible before slowing down near the target torque.
It is further object of this invention to provide such a method and
apparatus which accomplishes the foregoing with a minimum of user
involvement and complexity.
The present invention provides a method and apparatus for
correcting torque overshoot in power tools wherein torque per angle
rate is sensed on each rundown and the speed of the tool is
automatically adjusted to maintain torque accuracy on higher torque
rate joints while maintaining speed on lower torque rate joints,
automatically minimizing tool heating and maximizing job rate. In
particular, in the setting of threaded fasteners, joint rates are
defined as high (hard), medium or low (soft). Two torque points are
defined, a first torque point and a second torque point, which are
percentages of the target torque, for example 25% and 50%
respectively. Two angle values are defined corresponding to the
number of degrees of tool spindle rotation measured along the
portion of the torque/angle curve between the first torque point
and the second torque point on typical linear hard and medium
joints. Two final tightening speeds are defined as percentages of
full speed corresponding to the tool system requirements for
accurate shutoff on the two types of joints, hard and medium. A
controller counts the number of angle degrees between the first
torque point and the second torque point. The angle counted is
compared with the values of hard and medium angles to determine the
joint rate. If the angle is less than or equal to the hard angle,
the joint is considered hard and the controller immediately
downshifts the tool to the hard speed. If the angle is greater than
the hard angle but less than or equal to the medium angle, the
joint is considered medium and the controller immediately
downshifts the tool to the medium speed. If the angle is greater
than the medium angle, the joint is considered soft and the speed
is not changed.
The foregoing and additional advantages and characterizing features
of the present invention will become clearly apparent upon a
reading of the ensuing detailed description together with the
included drawing wherein:
BRIEF DESCRIPTION OF THE DRAWING FIGURES
FIGS. 1A and 1B are schematic diagrams of an electric power tool
according to the present invention;
FIG. 2 is a block diagram further illustrating the electric power
tool of FIG. 1;
FIG. 3 is a graph including a torque-angle plot illustrating an
aspect of the present invention;
FIGS. 4A, 4B and 4C are graphs including speed-time plots further
illustrating the present invention; and
FIG. 5 is a structure chart of a program for carrying out the
present invention.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
FIGS. 1A and 1B illustrate an electric power tool in the form of a
portable d.c. electric nutsetter and associated controller
incorporating the control system and method to correct for torque
overshoot according to the present invention. The power tool 10
shown is a rotary power tool for use in setting screws, nuts and
other threaded fasteners. Typically, an electric fastening tool
comprises a hand grip 12, control switch operator 14, high speed
electric motor 16, gear reduction 18 to reduce speed and increase
torque and an output drive 20 to connect to a fastener. There is
also provided a resolver 24 for motor commutation and for providing
angle information and a transducer 26 for providing torque
information, all in a manner which will be described. The output
drive 20 is provided with a key 30 for connection to a socket 36
for application to a fastener for tightening the same. FIG. 1A also
illustrates a typical fastening job wherein sheets 40, 42 of a
workpiece are joined by fastener 44 comprising threaded bolt 46 and
nut 48.
A cable 54 connects tool 10 to a control housing 56 containing a
power supply, servo amplifier and microprocessor-based controller
(not shown in FIG. 1), all of which will be described in further
detail presently. Cable 54 includes a plurality of conductors (not
shown in FIG. 1) for supplying electrical power to motor 16 for
operating the same, and for connecting resolver 24 and transducer
26 to the above-mentioned controller for a purpose to be described.
As shown in FIG. 1A, housing 56 is provided on one side with a
connector 60 for providing electrical connection to the a-c line, a
ground fault interrupter 62, a connector 64 to receive the end of
cable 54, an I/O connector 66, and communication ports 68 and 70
whereby the system can be operatively connected to a printer or bar
code reader and to a host computer, respectively. As shown in FIG.
1B, housing 56 is provided on the front side with a keypad 74 for
inputing commands and information to the afore-mentioned controller
and a display 76. The front side of housing 56 also includes a main
on-off switch 78 and indicator lamps generally designated 80 for
signalling various modes of operation.
The system of FIG. 1 is illustrated further by the schematic block
diagram of FIG. 2. As described in connection with FIG. 1, tool 10
includes a motor 16, speed reducing gears 18 coupling the motor
output shaft to an output drive 20, a torque transducer 26
operatively connected between the gears 18 and output drive 20 and
a resolver 24 operatively associated with motor 16. Motor 16 is a
brushless, electronically commutated, resolver-based DC servo motor
which runs on pulse width modulated (PWM) direct current and has a
stator provided with a three phase stationary winding and a
plurality of rare earth magnets bonded to the rotor, for example
eight neodymium magnets providing an 8-pole motor. Resolver 24 is a
variable reluctance brushless resolver which provides positional
information for motor commutation and angle. Resolver 24 is a
rotating transformer which generates signals which are utilized to
commutate the motor windings and make the motor 16 run. These
resolver signals also are converted into angle information,
eliminating the need for a separate encoder on tool 10.
The signal output of resolver 24 is applied via line 88 to an input
of a servo amplifier designated 90 in FIG. 2. The signals from
resolver 24 are used by servo amplifier 90 to commutate motor 16,
and the resolver signals are converted by servo amplifier 90 into
angle information relating to the rotation of tool 10. In
particular, servo amplifier 90 includes an isolation transformer,
full wave bridge rectifier, filter and amplifier circuit. Servo
amplifier 90 decodes the resolver signals generated by the dc servo
motor 16 in tool 10 and uses this feedback information to rim the
motor in a closed velocity loop. The amplifier 90 switches the
voltage in the three phase windings of motor 16 to maintain the
rotational speed and direction commanded by the fastening system
controller. A filtered power supply 96 operatively associated with
servo amplifier 90 converts 115 or 230 VAC single-phase line
voltage to a DC bus voltage which it supplies to amplifier 90. The
servo amplifier, in turn, provides this DC bus voltage to the tool
spindle motor 16 while monitoring the rotor position via resolver
24 as described.
A microprocessor-based controller 104 is provided which utilizes
the built-in torque transducer 26 of tool 10 to control peak
dynamic torque, i.e. the peak torque measured while the tool 10 is
setting a threaded fastener, and to stop tool 10 when it achieves a
torque preset on the controller. Resolver 24 in tool 10 also serves
as an angle encoder, allowing the tool to be used with controller
104 to measure the number of degrees of rotation of the tool
spindle above a torque threshold. This measured angle can be used
for monitoring or for tool shutoff from a preset angle. Thus,
controller 104 is connected in controlling relation to servo
amplifier 90 via path 108 for controlling the speed of tool 10.
Signals containing torque and angle information from transducer 26
and resolver 24, respectively, are sent to controller 104 via paths
110 and 112, respectively. Controller 104 also has two
communication ports, one designated 118 for use with a printer, bar
code reader or host computer 120 and the other designated 122 for
connection to a network 124 for uploading and downloading to a
personal computer.
In the operation of the arrangement of FIGS. 1 and 2, in response
to manual actuation of switch operator 14, rotational output from
tool 10 is transmitted through output drive 20 and socket 36 to
fastener 44 for tightening the same in a known manner. During the
fastening job, an analog signal from torque transducer 26 is
applied to controller 104 in which a torque target value has been
set previously by the operator using keypad 74. When that target is
reached, a control signal is sent from controller 104 to servo
amplifier 90 to stop motor 16 and shut the tool off. In this
connection, servo amplifier 90 can apply negative torque to brake
the motor 16.
In the foregoing basic operation as described, the tool stops, but
not quickly enough to prevent some torque overshoot above the
torque set on the controller. The time required to sense the
torque, process the information and remove the run signal, along
with the inertia in the decelerating high-speed elements in the
tool, cause the rotating tool components to continue to rotate
beyond the point at which they were to have stopped. This excess
rotation, when transmitted through the tool to the fastener, can
potentially drive the final fastener torque well beyond the target
torque set in the controller.
In accordance with the present invention, there is provided a new
and improved control system and method to correct for torque
overshoot in such electric power tools. Controller 104 has an
automatic downshift feature which senses torque rate on each
rundown and automatically adjusts the speed of motor 16 to maintain
accuracy on hard joints while maintaining speed on soft joints.
This reduces fastening cycle time and minimizes tool heating
automatically. To accomplish the foregoing, controller 104 takes an
angle snapshot between two torque points during the linear portion
of each rundown. The angle counted is compared to preset angles and
controller 104 determines if the joint is hard, medium or soft. If
the joint is hard or medium, controller 104 downshifts tool 10 to
one of two new lower final rundown speeds. If the joint is
considered soft, controller 104 maintains maximum speed. This
allows tool 10 to run at maximum speed as long as possible before
slowing down near the target torque.
Referring now to FIG. 3, curve 130 is a typical torque vs. angle of
mm plot for operation of tool 10 while setting a threaded fastener.
In accordance with the present invention, two torque points are
defined, a first torque point 132 and a second torque point 134,
which are percentages of the target torque 136, for example 25% and
50%, respectively. Two angle values are defined corresponding to
the number of degrees measured along the portion of curve 130
between first torque point 132 and second torque point 134 on
typical linear hard and medium joints. In particular, an automatic
downshift hard angle parameter sets the maximum hard joint angle
value, in degrees, for the automatic downshift software in
controller 104, and an automatic downshift medium angle parameter
sets maximum medium joint angle value, in degrees, for the
automatic downshift software. Two final tightening speeds for tool
10 are defined as percentages of full speed corresponding to the
tool system requirements for accurate shutoff on the two types of
joints, hard and medium. In particular, an automatic downshift hard
speed percent parameter sets the tool speed desired after the
automatic downshift software determines that the joint is hard, and
an automatic downshift medium speed percent parameter sets the tool
speed desired after the automatic downshift software determines the
joint is medium.
In accordance with the present invention, when the torque as
measured by transducer 26 reaches the level of automatic downshift
torque point 132, i.e. the first torque point, controller 104
begins counting degrees of tool spindle rotation, i.e. angle, as
measured by resolver 24. When the torque reaches the level of
automatic downshift torque point 134, i.e. the second torque point,
controller 104 compares the angle counted with the values of the
automatic downshift hard and medium angles to determine the joint
type, i.e. hard, medium or soft. The joint is considered soft
unless the angle counted is below the automatic downshift medium
angle. The joint is then considered medium unless the angle counted
is also below the automatic downshift hard angle. The joint is then
considered hard.
As soon as controller 104 determines that the joint is hard or
medium, it immediately causes the speed of tool 10 to change to the
percentage of maximum tool speed defined by the automatic downshift
hard or medium percent parameters. For soft joints the tool speed
is not changed.
The foregoing is illustrated further in FIGS. 4A, 4B and 4C which
are graphs including speed/time curves which show effects on
various parameters with the three different joint rates, i.e. hard,
medium and soft. In particular, curves 144A, 144B and 144C are
speed/time curves for hard, medium and soft joints, respectively.
Portions 146A 146B and 146C represent jog speed of tool 10,
portions of 148A, 148B and 148C represent tool ramp-up speed and
portions 150A, 150B and 150C represent a percent of maximum tool
speed. The automatic downshift feature of the present invention is
illustrated by the speed shift points 152A and 152B for the hard
and medium joints and by the lack of speed shift point for the soft
joint. Referring first to curve 144A, when controller 104
determines that the joint is hard, the speed of tool 10 is shifted
at point 152A from the level 150A down to the level 154A for hard
joints. The tool speed remains at this low level until the tool
shutoff point 156A is reached. On the other hand, when controller
104 determines that the joint is medium, the speed of tool 10 is
shifted at point 152B from the level 150B down to the level 154B
for medium joints. The tool speed remains at this low level until
the tool shutoff point 156B is reached. It can be seen from FIGS.
4A and 4B that the downshift speed 154A is lower for hard joints
than the downshifted speed 154B for medium joints. Lastly, when
controller 104 determines that the joint is soft, the speed of tool
10 remains at the upper level 150C until the tool shutoff point
156C is reached. Thus, by virtue of the present invention accurate
tool shutoff is provided for all three types of joints and the
problem of torque overshoot is avoided.
The system of the present invention is illustrated further by FIG.
5 which is a structure chart of the principal modules in the
portion of a typical microprocessor 170 of controller 104 for
carrying out the present invention. The instantaneous torque
obtained from transducer 26 is applied as an input to each of the
program modules 172 and 174 which determine the time at which the
selected fractional values of target torque occur, for example 25%
and 50%, respectively, in the present illustration. In this
connection, another module 176 which contains the target torque set
on the meter applies that target torque to another input of each of
the modules 172 and 174. Thus when 25% of target torque is reached
module 172 provides an output, and when 50% of target torque is
reached module 174 provides an output. A counter module 180 counts
the number of angle degrees obtained from the output of resolver 24
and under control of the outputs from modules 172 and 174. In
particular, the signal output of resolver 24 comprises pulses
containing information as to the number of degrees of tool spindle
rotation. When 25% of target torque is reached, module 172 provides
an output signal which causes counter module 180 to begin counting
the number of degrees of tool spindle rotation received via the
signal from resolver 24. When 50% of target torque is reached,
module 174 provides an output signal which causes counter module
180 to stop counting and provide an output signal containing the
number of angle degrees between the selected fractional values of
target torque.
A first angle module 182 contains an angle quantity corresponding
to that of a high or hard rate joint. A first comparator module 184
compares the angle output of counter module 180 with the angle
quantity of module 182, and if angle quantity of module 180 is less
than or equal to the angle quantity of module 182, module 184
provides an output indicating that the joint is of the high or hard
rate type. The output of comparator module 184 is used by a first
speed control module 186 which provides a signal to servo amplifier
90 to cause a downshift of the tool speed to the level for high or
hard rate joints, i.e. the speed level designated 154A in FIG.
4A.
Similarly, a second angle module 192 contains an angle quantity
corresponding to that of a medium rate joint. A second comparator
module 194 compares the angle output of counter module 180 with the
angle quantity of module 192, and if the angle quantity of module
180 is less than or equal to the angle quantity of module 192,
module 194 provides an output indicating that the joint is of the
medium rate type. The output of comparator module 194 is used by a
second speed control module 196 which provides a signal to servo
amplifier 90 to cause a downshift of the tool speed to the level
for medium rate joints, i.e. the speed level designated 154B in
FIG. 4B.
A third comparator module 198 compares the angle output of counter
module 180 with the angle quantity of module 192, and if the angle
quantity of module 180 is greater than the angle quantity of module
192, module 198 provides an output indicating that the joint is of
the low rate or soft type. The output of module 198 is received by
a module 200 which results in no change being made in the tool
speed.
The lines 202, 204 and 206 leading from the outputs of comparator
modules 184, 194 and 198, respectively, illustrate the capability
of utilizing the indications of hard, medium and soft joints,
respectively, for whatever additional purposes may be of
interest.
By way of example, in an illustrative tool system according to the
present invention, controller 104 is commercially available from
Stanley Air Tools, Cleveland, Ohio under the designation Series
T801 and incorporates an Intel 8088 microprocessor, servo amplifier
90 under the designation X5389, power supply 96 consisting of
transformer R7706, ground fault circuit interrupter R7747,
rectifier R7752 and filter board X5392. Motor 16 and resolver 24
are commercially available from Stanley Air Tools, Cleveland, Ohio
under the designation N4194. Torque transducer 26 is commercially
available from Stanley Air Tools, Cleveland, Ohio under the
designation N4245.
It is therefore apparent that the present invention accomplishes
its intended objects. There is provided a system and method to
correct for torque overshoot in electric power tools which operates
efficiently and effectively on high, medium and low torque rate
fastening jobs. By sensing torque per angle rate on each rundown
and automatically adjusting the tool speed to maintain torque
accuracy on higher torque rate joints while maintaining speed on
lower rate joints, the tool is allowed to run at maximum speed as
long as possible before slowing down near the target torque. This,
in turn, automatically minimizes tool heating and maximizes job
rate. The foregoing is accomplished with a minimum of user
involvement and complexity.
While an embodiment of the present invention has been described in
detail, that is for the purpose of illustration, not
limitation.
* * * * *